Explain Active Transport - Mechanism of Urine Formation

Active transport: When the substances are transported through a cell membrane against a concentration gradient, that is, from a dilute solution to a concentrated solution, energy must be imported to the molecules. This process of transport is called as the active transport. Active transport of sodium occurs from inside the epithelial cells into the spaces between the cells.

This transport from the cell diminishes the sodium concentration inside the cell. Other substances, besides sodium, that are actively absorbed through the tubular epithelial cells include glucose, amino acids, phosphate ions and others. Hydrogen ions and potassium ions are actively secreted into all or some portions of the tubules. Active secretion occurs in the same way as active absorption. Here, the cell membrane transports the secreted substances in the opposite direction i.e. from a low concentration to a high concentration, against a gradient.